Added option to manually select the syntax highlighting language

[albert-tomanek]

This is useful if you're editing an unsupported language.
(also added support for the NuShell scripting language)

By the way, I've made a backup of the only HTML-based documentation for TVision that I could find (in case it goes offline). You might want to clone it to your account since you host all the TVision-related stuff.
https://github.com/albert-tomanek/tvision-docs/
https://htmlpreview.github.io/?https://raw.githubusercontent.com/albert-tomanek/tvision-docs/master/html/index.html

[albert-tomanek]

I've also attempted to add support for saving turbo's settings to a config file.
The file looks like this:

[Editor]
lineNumbers = false
autoIndent = true
wrapping = true

[State]
mostRecentDir = "/home/user/github/turbo"

and is saved in ~/.config/turbo.toml.

I chose to use the libtomlcpp (a wrapper around tomlc99) library to parse the config file, because it seemed the most lightweight. Unfortunately I found out too late that it doesn't support writing toml files so I just fprintf the config file myself when saving. In the worst case we can switch toml libraries.

[OscarL]

Would be nicer not to hard-code "./config" here. Using $XDG_CONFIG_HOME would make it better for several OSes at least.